The investigations proposed here seek to determine whether prey-killing (e.g., mouse killing) by a number of mammalian species is controled by similar mechanisms in the various species. Pharmacological manipulations are used for evaluating the similarity of mechanisms controlling prey-killing in species to be compared. Effects of drugs which inhibit or enhance mouse killing by rats are used as standards for comparison. These drug effects are compared with drug effects on mouse killing by other species (e.g., cats, ferrets, grasshopper mice, guinea pigs, etc.), and with drug effect on the killing of other prey (e.g., frogs, grasshoppers, young rats, etc.). Attempts at delineation of the loci of central nervous and somatic mechanisms on which effective drugs act are carried out as well, in order to provide further specification of the degree to which various prey-killing control mechanisms may be similar.